KR20140092343A - Hydraulic drive system for work vehicle - Google Patents

Abstract

SUMMARY OF THE INVENTION An object of the present invention is to provide a hydraulic control apparatus for a hydraulic control apparatus which can supply pressurized oil to an accumulator when a work device is lifted, And to provide a hydraulic drive apparatus for a work vehicle which can reliably prevent a drop of a working device due to leakage from the oil tank to a gap through a gap.
The discharged oil of the main pump 13 is led to the accumulator 40 through the accumulator valve 61 when the working device 2 is raised. The pressure oil in the bottom chamber 11a of the lift arm cylinder 11 is prevented from flowing into the clearance of the control valve 20 for the lift arm when the working apparatus 2 is held in an elevated state To the working oil tank (17) by the poppet (71) of the load holding valve (70).

Description

HYDRAULIC DRIVE SYSTEM FOR WORK VEHICLE [0001]

The present invention relates to a hydraulic drive apparatus for a work vehicle that restrains up / down movement of a work device during load travel by connecting a bottom seal of a hydraulic cylinder supplied with compressed oil to an accumulator when raising a work device.

The hydraulic driving apparatus of a conventional working vehicle is disclosed in Patent Document 1. This hydraulic drive apparatus will be described next. Here, the terms described in Patent Document 1 are shown in " () ".

A hydraulic cylinder (boom cylinder) for supplying pressurized oil to the bottom chamber to raise the working device (boom, bucket) of the working vehicle (wheel loader) and supply the pressurized oil to the load chamber to lower the working device; A main pump (hydraulic pump) for discharging the driving oil to be driven; and a control valve for controlling the flow of the pressure oil supplied from the main pump to the hydraulic cylinder.

The control valve is a spool valve, and has a bottom chamber side port connected to the bottom chamber of the hydraulic cylinder via a bottom chamber side channel, a rod chamber side port connected to the load chamber of the hydraulic cylinder via a rod side channel, And a tank port connected to the working oil tank. The control valve has a first operating position, which is a valve position for connecting the pump port to the bottom actual port and also connects the rod actual port to the tank port, and a second operating position for connecting the pump port to the rod actual port, To a neutral position for shutting off both the second operating position, which is a valve position connecting to the port, between the pump port and the bottom actual port, and both the pump port and the rod actual port.

The conventional hydraulic drive apparatus includes an accumulator connected to an end of a connecting pipe branching from a bottom chamber side pipe, an opening / closing valve provided on the connecting pipe, and an accumulator for connecting the both ends of the accumulator to the connecting pipe, And an accumulation valve (a switching valve) is provided on the path conduit in parallel with the opening / closing valve.

The on-off valve is configured to be switchable between an open position, which is a valve position for connecting the bottom chamber and the accumulator, and a closed position, which is a valve position for shutting off the gap between the bottom chamber and the accumulator. The load is controlled to the open position.

The accumulator valve permits the flow of pressurized oil toward the accumulator in the accumulator for bypassing the accumulator, closes the accumulator pressure position which is the valve position for blocking the flow of the pressure oil in the opposite direction, Which is a valve position for shutting off the valve and the accumulator. In this accumulator valve, the oil pressure of the main pump is supplied to the bottom chamber through the control valve and the bottom chamber side channel, and the valve position is controlled to the accumulator pressure position when the pressure in the accumulator is less than the predetermined pressure .

In the conventional hydraulic drive apparatus constructed as described above, the open / close valve is controlled to the open position when the working vehicle performs the load travel. Thereby, the bottom chamber of the hydraulic cylinder is connected to the accumulator. In this state, the load pressure acting on the bottom chamber of the hydraulic cylinder during the load traveling is absorbed by the accumulator. As a result, the vertical movement of the working device of the working vehicle is suppressed.

When the valve position of the open / close valve is controlled to the open position, pressure oil flows into the accumulator from the bottom chamber of the hydraulic cylinder until the pressure in the accumulator becomes equal to the pressure in the bottom chamber of the hydraulic cylinder. The amount of the applied pressure oil is contracted. In order to reduce the amount of contraction of the hydraulic cylinder at this time, that is, the amount of descent of the working device, the accumulator valve is provided in parallel with the opening / closing valve.

That is, when the valve position of the accumulator valve is controlled to the axial pressure position, when the discharge oil of the main pump is supplied to the bottom chamber of the hydraulic cylinder through the control valve and the bottom chamber side channel in order to raise the working device, Is led to the accumulator through the bypass pipeline for the accumulation of pressure, and is accumulated in the accumulator. Therefore, when the work vehicle is to be loaded, after the work device is lifted, the pressure oil is already accumulated in the accumulator. Thus, when the valve position of the open / close valve is controlled to the open position, the amount of pressure oil flowing into the accumulator from the bottom chamber of the hydraulic cylinder can be reduced, and accordingly, the shrinkage amount of the hydraulic cylinder, .

Japanese Patent Application Laid-Open No. 2006-144248

In the above-described conventional hydraulic drive apparatus, the control valve (operation valve) is a spool valve. In this control valve, there is a gap between the spool and the land where the spool slides so that the spool can slide with respect to the land. Accordingly, even when the valve position of the control valve is held at the neutral position, the gap between the bottom chamber of the hydraulic cylinder (boom cylinder) and the hydraulic oil tank is not strictly blocked. Therefore, depending on the magnitude of the load pressure acting in the bottom chamber of the hydraulic cylinder, the pressure oil leaks to the hydraulic oil tank through the clearance of the control valve, thereby contracting the hydraulic cylinder and lowering the working device. When the operator operates the valve position of the control valve to the neutral position, the intention of the operator is to hold the raised state of the working device, so that the working device is lowered in a state where the valve position of the control valve is at the neutral position, Which is against the intention of the operator.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described circumstances, and it is an object of the present invention to provide an accumulator which can supply pressurized oil to an accumulator when lifting a working device, And it is an object of the present invention to provide a hydraulic drive apparatus for a work vehicle which can reliably prevent a drop of a working device due to leakage of pressure oil in a bottom chamber of a hydraulic cylinder to a hydraulic oil tank through a gap of a control valve.

In order to achieve the above-mentioned object, a hydraulic drive apparatus for a working vehicle according to the present invention is configured as follows.

[1] A hydraulic drive apparatus for a work vehicle according to the present invention includes: a hydraulic cylinder for supplying pressurized oil to a bottom chamber to raise a working device of a working vehicle and supply pressure oil to the load chamber to lower the working device; A bottom chamber side port connected to the bottom chamber via a bottom chamber side channel, a rod chamber side port connected to the rod chamber via a rod side channel, a main chamber side port connected to the main chamber, A control valve including a spool valve having a pump port and a tank port connected to an operating oil tank, an accumulator connected to an end of a connecting pipe branched from the bottom chamber side channel, an opening / closing valve provided on the connecting pipe, And a bypass pipe connected to the connecting pipe for bypassing the on-off valve, Wherein the control valve has a first operating position that is a valve position connecting the pump port to the bottom chamber port and connecting the rod chamber port to the tank port, A second operating position that is a valve position for connecting the bottom chamber side port to the tank port and a second operating position for connecting both the pump port and the bottom chamber side port, Wherein the open / close valve includes an open position, which is a valve position for opening the connection channel and connecting the bottom chamber to the accumulator, and an open position for closing the connection channel to connect the bottom chamber and the accumulator Which is a valve position for shutting off the working apparatus, The valve position is controlled in the direction from the closed position to the open position when the working vehicle travels, and the valve position is controlled to the closed position when the working vehicle is running, An accumulator pressure position which is a valve position for permitting flow of pressure oil toward the accumulator in the bypass pipeline for the accumulation and a valve position for closing the bypass pipeline for blocking the pressure oil flow to the accumulator Wherein when the pressure in the accumulator is lower than a predetermined pressure, the pressure in the accumulator is lower than a predetermined pressure, and the pressurized oil is supplied to the bottom chamber through the control valve and the bottom chamber. And the valve position is controlled to the axial pressure position. A load retaining valve is provided on the bottom chamber, and the load retaining valve is provided between the control valve and the bottom chamber through the bottom chamber, when the valve position of the control valve is the first operating position, And when the control valve is in the second operating position, allowing the flow of pressurized oil from the bottom chamber to the control valve through the bottom chamber side conduit is permitted, and when the load operation is performed, Wherein the control valve is configured to allow the flow of pressurized oil toward the accumulator through the connection pipe from the chamber to the accumulator when the valve position of the control valve is at a neutral position, Characterized in that the flow of the pressure oil toward the piston is blocked by a poppet .

In the hydraulic drive device of the work vehicle described in this item [1], when the work device is raised, the valve position of the control valve is switched to the first operating position, whereby the pump port of the control valve, And the rod actual port is connected to the tank port. In this state, the load holding valve permits the flow of the pressure oil from the control valve toward the bottom chamber of the hydraulic cylinder. Therefore, the discharged oil of the main pump passes through the pump port of the control valve and the bottom chamber side port, and then passes through the bottom side pipeline and is supplied to the bottom chamber of the hydraulic cylinder. As a result, And is exhausted to the working oil tank through the working channel, the rod-side port of the control valve, and the tank port. As a result, the hydraulic cylinder is stretched to raise the working device.

In this way, when the working device is lifted, that is, when the discharge oil of the main pump is supplied to the bottom chamber through the control valve and the bottom chamber side channel, if the pressure in the accumulator is less than the predetermined pressure, Position. At this time, the accumulator valve permits the flow of the pressure oil toward the accumulator in the bypass pipeline for the accumulation, and prevents the flow of the pressure oil in the opposite direction. Therefore, a part of the pressure oil flowing in the bottom chamber can flow until the pressure in the accumulator reaches a predetermined pressure, or until the valve position of the control valve for the lift arm becomes out of the first operating position, And is supplied to the accumulator through the path duct. As a result, the accumulator accumulates the pressure oil. That is, the hydraulic drive apparatus of the work vehicle described in [1] can supply the pressurized oil to the accumulator when the work apparatus is lifted.

The valve position of the control valve is held at the neutral position so that the control valve can be moved between the pump port and the bottom chamber side port and between the pump port and the rod chamber side Both ports between the ports are held in an interrupted state. In this state, the load holding valve prevents the flow of pressure oil from the bottom chamber of the hydraulic cylinder to the control valve by the poppet. As a result, the hydraulic cylinder is retained in a stretched state and held in a raised state with the working device. That is, in the hydraulic drive system of the work vehicle described in [1], since it is the poppet that blocks the flow of pressure oil from the bottom chamber of the hydraulic cylinder toward the control valve, The descent of the working device due to leakage of the pressurized oil in the bottom chamber of the hydraulic cylinder to the hydraulic oil tank through the clearance of the control valve can be reliably prevented.

When the working device is lowered, the valve position of the control valve is switched to the second operating position, whereby the pump port is connected to the rod in the rod in the control valve, and the rod in the control valve is connected to the tank port . In this state, the load holding valve permits the flow of pressure oil from the bottom chamber of the hydraulic cylinder to the control valve. Therefore, the discharged oil of the main pump passes through the pump port of the control valve and the port of the rod chamber, passes through the rod chamber, and is supplied to the rod chamber of the hydraulic cylinder. As a result, And is discharged to the working oil tank through the working channel, the bottom chamber side port of the control valve, and the tank port. As a result, the hydraulic cylinder is contracted to lower the working device.

The valve position of the control valve is held in the neutral position when the working vehicle is on the load traveling path so that both the pump port and the bottom actual port and the both ports between the pump port and the rod actual port are blocked . In this state, the open / close valve is controlled to the open position, and the load holding valve permits the flow of pressure oil from the bottom chamber of the hydraulic cylinder to the accumulator through the connecting pipe. As a result, the load pressure acting on the bottom chamber of the hydraulic cylinder during the load traveling is absorbed by the accumulator, whereby the up and down movement of the working device is suppressed.

Further, when the valve position of the open / close valve is controlled to the open position, the pressure oil flows into the accumulator from the bottom chamber of the lift arm cylinder until the pressure in the accumulator becomes equal to the pressure in the bottom chamber of the lift arm cylinder, Shrink only by the amount of pressure applied to the accumulator. Since the part of the discharge oil of the main pump is supplied to the accumulator when the working device is lifted up as described above, when the lifting arm cylinder is extended and the working device is lifted, when the wheel loader is loaded, the pressure oil is already accumulated in the accumulator have. This makes it possible to reduce the amount of shrinkage of the lift arm cylinder, that is, the amount of reduction of the lift arm immediately after the valve position of the open / close valve is controlled to the open position.

[2] A hydraulic drive apparatus for a working vehicle according to the present invention is a hydraulic drive apparatus for a work vehicle described in [1], wherein the load holding valve includes a return spring for pressing the poppet in a direction to close the poppet, A first opening side pressure chamber for introducing a pressure in a direction in which the poppet opens against the elastic force of the return spring from the bottom side port side of the control valve and a second opening side pressure chamber for opening the poppet against the elastic force of the return spring Side pressure chamber for introducing the pressure in the direction of closing the poppet to the second open side pressure chamber for introducing the pressure from the bottom chamber side of the hydraulic cylinder and the hydraulic pressure for pressing the poppet in the direction for closing the poppet, And a closed-side pressure chamber which is introduced from between the bottom chamber side channel and the closed side of the load holding valve And a switching valve is interposed between the pressure chamber and the switching valve. The switching valve includes a first port connected between the load holding valve and the bottom chamber with respect to the bottom chamber side channel, and a second port connected to the closing valve side pressure chamber A second port, and a tank port connected to the working oil tank, the first port being connected to the second port and the poppet blocking the flow of the pressure oil from the first and second ports toward the tank port, Wherein the control valve is switchable between a supply position in which the first port is connected to the first port and a discharge position in which the second port is connected to the tank port when the valve position of the control valve is the neutral position, When the load travel is performed, and when the valve position of the control valve is lower than the second operation position And is controlled to the discharge position when the discharge opening is closed.

The operation relationship between the switching valve and the load holding valve in the hydraulic driving apparatus of the working vehicle described in this "[2]" will be described.

The valve position of the control valve is switched to the first operating position so that the pump port is connected to the bottom actual port and the rod actual port is connected to the tank port in the control valve . In this state, the valve position of the switching valve is controlled to the supply position. Therefore, the pressure in the bottom chamber of the hydraulic cylinder is introduced into the closed-side pressure chamber through the portion of the bottom chamber side channel interposed between the load holding valve and the bottom chamber and through the first port and the second port of the switching valve, Press with the spring in the direction to close the poppet. On the other hand, the discharge pressure of the main pump is introduced into the first opening side pressure chamber through the portion of the bottom chamber side channel interposed between the control valve and the load holding valve, and presses the poppet in the opening direction. The force that presses the discharge pressure of the main pump introduced into the first opening side pressure chamber in the direction of opening the poppet is the same as the pressure in the bottom chamber of the hydraulic cylinder introduced into the closed side pressure chamber and in the direction in which the return spring closes the poppet The poppet is pushed against the pressing force, whereby the load pressure holding valve is opened. That is, the load holding valve permits the flow of pressure oil from the control valve to the bottom chamber of the hydraulic cylinder through the bottom chamber side channel.

When the working device is lowered, the valve position of the control valve is switched to the second operating position, whereby the pump port is connected to the rod-side port of the control valve and the bottom-chamber-side port is connected to the tank port . In this state, the valve position of the switching valve is controlled to the discharge position. Accordingly, the pressure in the closed-side pressure chamber of the load holding valve is discharged to the working oil tank through the second port of the switching valve and the tank port. On the other hand, the discharge oil of the main pump passes through the pump port of the control valve and the port of the rod chamber, passes through the rod chamber, and is supplied to the rod chamber of the hydraulic cylinder. Subsequently, the pressure in the bottom chamber of the hydraulic cylinder is introduced into the second open side pressure chamber of the load retaining valve, pushing the poppet against the elastic force of the lift arm cylinder, whereby the load retaining valve is opened. That is, the load holding valve permits the flow of pressure oil toward the control valve through the bottom chamber of the hydraulic cylinder through the bottom chamber side channel.

The valve position of the control valve is held at the neutral position so that the control valve can be moved between the pump port and the bottom chamber side port and between the pump port and the rod chamber side Both ports between the ports are held in an interrupted state. In this state, the valve position of the switching valve is controlled to the supply position. Therefore, the pressure in the bottom chamber of the hydraulic cylinder is introduced into the closed-side pressure chamber through the portion of the bottom chamber side channel interposed between the load holding valve and the bottom chamber and through the first port and the second port of the switching valve, Press with the spring in the direction to close the poppet. The pressure in the bottom chamber of the hydraulic cylinder is transmitted through the portion of the bottom chamber side conduit interposed between the load holding valve and the bottom chamber of the hydraulic cylinder so that the pressure in the bottom chamber of the hydraulic cylinder becomes the second opening side pressure So that the pressure is applied in the direction of opening the poppet. Further, the discharge pressure of the main pump is not introduced into the first open side pressure chamber of the load holding valve since the load holding valve and the main pump are shut off by the control valve. The pressure in the closed-side pressure chamber and the return spring press against the pressure in the bottom chamber of the hydraulic cylinder introduced into the second open-side pressure chamber in the direction of closing the poppet, whereby the load pressure holding valve is closed . That is, the load holding valve prevents the flow of pressure oil from the bottom chamber of the hydraulic cylinder to the control valve through the bottom chamber side channel by the poppet.

The valve position of the control valve is held in the neutral position when the working vehicle is on the load traveling path so that both the pump port and the bottom actual port and the both ports between the pump port and the rod actual port are blocked . In this state, the valve position of the switching valve is controlled to the discharging position, and accordingly, the pressure in the closing side pressure chamber of the load holding valve is discharged to the working oil tank through the second port and the tank port of the switching valve. At this time, pressure in the bottom chamber of the hydraulic cylinder is introduced into the second open side pressure chamber of the load holding valve through a portion of the bottom chamber side channel interposed between the load holding valve and the bottom chamber of the hydraulic cylinder. Accordingly, the pressure in the bottom chamber of the lift arm cylinder introduced into the second open side pressure chamber pushes the poppet against the return spring, whereby the load holding valve is opened. On the other hand, since the open / close valve is controlled to the open position and the valve position of the control valve is set to the neutral position and the main pump is interrupted between the load holding valve and the hydraulic oil in the bottom chamber of the hydraulic cylinder, So that it is guided to the accumulator through the load holding valve and the connecting conduit. That is, the load holding valve permits the flow of pressure oil from the bottom chamber of the hydraulic cylinder to the accumulator through the connecting pipe.

Further, in the hydraulic drive apparatus for a work vehicle described in [2], when the work apparatus is lifted and the work apparatus is held in an elevated state except during load travel, The flow of the pressure oil from the first port toward the tank port and the flow of the pressure oil from the second port toward the tank port are inhibited so that the pressure in the bottom chamber of the hydraulic cylinder is not leaked to the closing- To act on the poppet. Therefore, the poppet of the load holding valve can be held in a securely closed state, thereby contributing to prevention of leakage of the pressure oil from the load holding valve to the control valve for the lift arm.

The hydraulic drive apparatus for a working vehicle according to the present invention can supply pressurized oil to the accumulator through the accumulator when the working apparatus is lifted. This makes it possible to reduce the lower limit of the working device immediately after the valve position of the open / close valve is controlled to the open position during the load traveling.

In the hydraulic driving apparatus for a working vehicle according to the present invention, when the working apparatus is held in an elevated state except when the load is running, the pressure oil in the bottom chamber of the lift arm cylinder is pressurized It is possible to reliably prevent the work device from dropping due to leakage from the working oil tank to the working oil tank by the poppet of the load holding valve. Thereby, it is possible to prevent the working device from descending against the intention of the operator.

1 is a side view of a wheel loader which is a working vehicle according to an embodiment of the present invention.
2 is a hydraulic circuit diagram showing a hydraulic drive apparatus according to an embodiment of the present invention.
3 is a characteristic diagram showing the relationship between the control current applied to the opening / closing control valve shown in Fig. 2 and the opening degree of the opening / closing valve.
4 is a characteristic diagram showing the relationship between the elapsed time in a state in which the vehicle speed falls within the specified range of the vehicle speed at the time of vehicle load and the current value applied to the opening / closing control valve.
5 is a flowchart showing a process performed by the controller shown in Fig. 2 when controlling the opening / closing control valve.

A hydraulic drive apparatus for a work vehicle according to an embodiment of the present invention will be described with reference to Figs. 1 to 5. Fig.

The hydraulic drive apparatus according to the present embodiment is applied to, for example, the work vehicle shown in Fig. 1, that is, the wheel loader 1. The working device 2 of the wheel loader 1 includes a lift arm 3 provided on a front portion of the vehicle body so as to be vertically rotatable and a bucket 3 which is vertically rotatable on the front end of the lift arm 3, (4). The lift arm 3 and the bucket 4 are respectively driven by the lift arm cylinder 11 (hydraulic cylinder) and the bucket cylinder 12 (hydraulic cylinder), respectively.

2, the hydraulic drive system 10 according to the present embodiment is provided with a lift arm cylinder 11, a bucket cylinder 12, and a lift arm cylinder 11 and a bucket cylinder 12 And the pressure supplied from the main pump 13 to the lift arm cylinder 11 between the main pump 13 and the lift arm cylinder 11 A bucket cylinder 12 for controlling the flow of the pressure oil supplied from the main pump 13 to the bucket cylinder 12 between the main pump 13 and the bucket cylinder 12; And a control valve 25 for controlling the operation of the engine. The control valve 20 for the lift arm and the control valve 25 for the bucket are provided on the bypass pipe 14 and are connected to each other in parallel with the bypass pipe 14. [

The lift arm cylinder 11 is a single rod type double acting cylinder and includes a bottom chamber 11a which is a pressure chamber formed on the bottom side of the cylinder tube and a rod chamber 11b which is a pressure chamber formed on the rod side of the piston. The lift arm cylinder 11 elevates the bucket 4 by rotating the lift arm 3 upward by supplying pressurized oil to the bottom chamber 11a so as to supply the pressurized oil to the load chamber 11b, The lift arm 3 is rotated in the downward direction to lower the bucket 21a. Hereinafter, the lift arm 3 is rotated in the upward direction to raise the bucket 4 is referred to as the lift of the working device 2, and the lift arm 3 is rotated in the downward direction to lower the bucket 4 It is referred to as the descent of the device 2. [

The lift arm control valve 20 includes a spool valve and includes a bottom chamber side port 21 connected to the bottom chamber 11a of the lift arm cylinder 11 through a bottom chamber side channel 15, A load chamber side port 22 connected to the load chamber 11b of the pump chamber 11 via a rod 16 on the side of the load chamber, a pump port 23 connected to the main pump 13, And has a tank port (24). The control valve 20 for the lift arm is a spring center type three-position valve of a hydraulic pilot type. The pump port 23 is connected to the bottom chamber side port 21 and the rod chamber side port 22 is connected to the tank port 2 and the pump port 23 are connected to the rod side port 22 and the bottom side port 21 is connected to the first working position 20a (the valve position on the right side in Fig. The second operating position 20b (valve position on the left side in Fig. 2) which is a valve position for connecting the pump port 23 to the tank port 24 and the pump port 23 and between the pump port 23 and the bottom- And a neutral position 20c (valve position in the center of FIG. 2) for blocking both sides between the rod side ports.

The hydraulic drive system 10 further includes a pilot pump 30 and an operation lever device 31 for generating a pilot pressure from the discharge pressure of the pilot pump 30. [ The operation lever device 31 includes an operation lever 31a which can be hardened in two directions opposite to the initial position. When the operation lever 31a is hardened in one direction from the initial position, A pilot pressure for elongation is generated which switches the valve position of the valve 20 from the neutral position 20c to the first operating position 20a and the operating lever 31a is operated in the other direction opposite to the one direction , A pilot pressure for shrinking which changes the valve position of the control valve 20 for the lift arm from the neutral position 20c to the second operating position 20b is generated. The pilot pressure for elongation is guided to the control valve 20 for the lift arm by a pilot channel 32 for elongation and the pilot pressure for retraction is guided to the control for lift arm And is guided to the valve 20.

The bucket cylinder 12 is a single-rod type double acting cylinder as the lift arm cylinder 11. The control valve 25 for the bucket is a spool valve similar to the control valve 20 for the lift arm, and is a spring center type three-position valve in a hydraulic pilot type. The connection relationship between the control valve 25 for this bucket and the bucket cylinder 12 is the same as the connection relationship between the control valve 20 for the lift arm and the lift arm cylinder 11. [ An operation lever device for generating a pilot pressure for the control valve 25 for the bucket is not shown.

The hydraulic drive system 10 further includes an accumulator 40 connected to the end of the connecting pipe 41 branched from the bottom chamber side conduit 15 and extending from the load chamber side conduit 16 to the working oil tank 17 And a return pipe 18 provided on the connecting pipe 41 and the return pipe 18 so as to open and close the connecting pipe 41 and the return pipe 18 to open and close the valve. The open / close valve 50 is a spring return type two-position valve of a hydraulic pilot type and is disposed between the closed position 50a (the valve position on the right side in Fig. 2) and the open position 50b So that the valve position can be changed. The closed position 50a is a normal position defined by the return spring 51 and is a valve position for closing both the connecting pipe 41 and the return pipe 18. [ The open position 50b is an operating position and is a valve position for opening both the connecting pipe 41 and the return pipe 18. [ The pilot pressure in the direction in which the open / close valve 50 is opened is applied to the pressure receiving surface 52 of the open / close valve 50 through the pilot pipe 53 for the open valve. The drain pipe 54 extends from the return spring 51 side of the opening / closing valve 50 and is connected to the return pipe 18.

The hydraulic drive system 10 further includes a spring return type electromagnetic proportional valve connected to the pilot pipe 53 for the open valve and a pilot pressure applied to the pressure receiving surface 52 of the open / Closing control valve 55 which is generated from the discharge pressure of the valve body. The opening / closing control valve 55 is capable of varying the valve position between the normal position 55a and the operating position 55b and is capable of changing the valve position from the normal position 55a to the operating position 55b, As shown in FIG.

The hydraulic drive system 10 further includes a vehicle speed sensor 56 for detecting the vehicle speed of the wheel loader 1 and outputting a vehicle speed detection signal (electric signal) corresponding to the vehicle speed, A controller 57 for controlling the open / close control valve 55 based on the vehicle speed detection signal and an instruction switch (not shown) for outputting an instruction signal corresponding to an instruction to operate the open / close control valve 55 58 (instruction SW).

When the instruction signal from the instruction switch 58 is input, the controller 57 determines whether or not the vehicle speed has fallen within the specified range at the time of vehicle loading. If the result of the determination is that the vehicle speed is within the specified range, And is set to operate by supplying a control current to the control valve 55. The specified range of the vehicle speed at the time of loading is preset as a range of the vehicle speed suitable for running (load driving) in a state in which the load is loaded on the bucket 4.

The current value I of the control current applied to the opening / closing control valve 55 is calculated by a CPU, a ROM, and a RAM provided in the controller 57. 3, the relationship between the current value I of the control current and the opening degree (opening / closing valve opening degree) of the opening / closing valve 50 is such that the opening / closing of the opening / closing valve 50 Is set to be large. The valve body of the opening / closing valve 50 is provided with a notch (not shown), and the first range in which the range of the current value I of the control current is set to "I0? I? I1" The opening degree of the opening 50 is changed. In the second range in which the control current range is set to " I1 < I < Imax ", the opening degree of the opening / closing valve 50 is changed in a range exceeding the notch position. The change rate of the opening degree of the opening / closing valve 50 in the first range is set to be smaller than the opening degree change rate of the opening / closing valve 50 in the second range due to the cutting, The opening degree of the opening / closing valve 50 can be finely adjusted more than the range.

4, the relationship between the vehicle speed of the wheel loader 1 and the current value I of the control current is such that when the vehicle speed is within the specified range at the time of vehicle loading, During the elapse of T1, the control current I increases in proportion to the elapsed time and is set to reach the current value I1.

The controller 57 is set to control the opening / closing control valve 55 in the order shown in Fig. That is, when the instruction switch 58 is turned on and the instruction signal is input to the controller 57 ("YES" in step S1), the controller 57, based on the vehicle speed detection signal from the vehicle speed sensor 56, It is determined whether or not the vehicle speed of the vehicle is within the specified range at the time of the load driving (step S2). As a result of the determination, when the vehicle speed is within the specified range of the vehicle speed at the time of loading (step S2, YES), the control current is supplied to the opening / closing control valve 55 (step S3). The routine of these steps S1 to S3 is repeated as long as the state in which the vehicle speed is within the specified range at the time of the load driving in the state that the indicating switch 58 is ON and the valve position of the opening / And the state in which the direction is changed from the open position 50a to the open position 50b is held.

If the instruction switch 58 is off ("NO" in step S1) or if the vehicle speed is not within the specified range at the time of vehicle loading ("NO" in step S2), the controller 57 controls the opening / (Step S4). Therefore, when the indicating switch 58 is turned off or the vehicle speed continues to fall outside the specified range at the time of loading, the opening / closing valve 50 is controlled to the closing position 50a by the return spring 51. [

That is, the opening / closing valve 50 is closed by the pilot pipe 53 for the open valve, the pilot pump 30, the opening control valve 80, the indicating switch 58, the vehicle speed sensor 56, The valve position of the on-off valve 50 is controlled to the open position 50b when the wheel loader 1 carries the load. Further, when the vehicle is not in the load traveling state, the return spring 51 is controlled to the closing position 50a.

The hydraulic drive apparatus 10 is further provided on an axial-pressure bypass line 60 which is connected to the connection line 41 at both ends thereof and is located above the open-close valve 50, 61). The accumulator valve 61 is a spring return type two-position valve of a hydraulic pilot type, and an axial pressure position 61a and a closed position 61b are set as valve positions. The accumulating pressure position 61a is a normal position defined by the return spring 62 and permits the flow of the pressure oil toward the accumulator 40 in the accumulation bypass conduit 60 by the check valve 63 , And a valve position for stopping the flow of the pressure oil in the opposite direction. The closed position 61b is an operating position and is a valve position for closing the bypass pipeline 60 for the pressure accumulation to block the gap between the control valve 20 for the lift arm and the accumulator 40. [

An end of the bypass line 60 for the axial compression which is connected to the side of the bottom chamber side conduit 15 with respect to the opening and closing valve 50 of the connection line 41 is connected to the end of the axial pressure valve 61 , An throttling valve 64 is provided. A pilot line 66 for stopping axial pressure is provided from a portion between the throttling valve 64 and the axial pressure valve 61 to apply the pressure of this portion to the hydraulic pressure surface 65 of the axial pressure valve 61 as a pilot pressure . The pressure in the accumulator 40 is applied to the pressure receiving surface 65 of the accumulator valve 61 by the throttling valve 64 and the pilot line 66 for stopping the accumulation. The drain line 67 extends from the return spring 62 side of the accumulator 61 and is connected to the return line 18.

The accumulator 40 is controlled such that the accumulator 40 is in a state where the pressure in the accumulator 40 is less than a predetermined pressure and the valve position is controlled to the accumulating pressure position 61a. The predetermined pressure is defined by the set load of the return spring 62. It is assumed that the load pressure acting on the bottom chamber 11a of the lift arm cylinder 11 at the time of load travel is assumed and the load pressure is applied to the accumulator 40 The valve position of the accumulator 61 is set to be switched to the closed position 61b.

The hydraulic drive system 10 further includes a load holding valve 70 on the bottom chamber side conduit 15. The load holding valve 70 is a spring return type poppet valve for opening and closing the bottom chamber side conduit 15 and includes a poppet 71, a valve seat 72 which is in contact with the poppet 71, A return spring 73 for urging the poppet 71 in a direction toward the valve seat 72 or closing the valve seat 72 and a pressure in the direction in which the poppet 71 opens against the elastic force of the return spring 73, The first open side pressure chamber 74 introduced from the bottom valve side port 21 side of the control valve 20 and the pressure in the direction in which the poppet 71 opens against the elastic force of the return spring 73, The second open side pressure chamber 75 which is introduced from the bottom chamber 11a side of the arm cylinder 11 and the oil pressure which presses the poppet 71 in the direction of closing the poppet 71, And a closed side pressure chamber 76 which is introduced from between the holding valve 70 and the bottom chamber 11a of the lift arm cylinder 11. The closing side pressure receiving surface 71a of the poppet 71 receiving the pressure in the closed side pressure chamber 76 is larger than the opening side pressure receiving surface 71b of the poppet 71 receiving the pressure in the second opening side pressure chamber 75 . The closed-side pressure chamber 76 also serves as a spring chamber that houses the return spring 73.

The hydraulic drive unit 10 further includes a switching valve 80 interposed between the bottom chamber side conduit 15 and the closed side pressure chamber 76 of the load holding valve 70. The switching valve 80 has a first port 81 connected to the bottom chamber side conduit 15 between the load holding valve 70 and the bottom chamber 11a of the lift arm cylinder 11, A second port 82 connected to the pressure chamber 76, and a tank port 83 connected to the working oil tank 17. The switch valve 80 is a spring return type two-position valve that connects the first port 81 to the second port 82 and also connects the first port 81 and the second port 82, Which is a valve position for connecting the second port 82 to the tank port 83 and a supply position 80a (normal position) which is a valve position for stopping the flow of the pressure oil toward the tank port 83 by the poppet 84, Position 80b (operating position). The return pipe 85 connecting the tank port 83 of the switching valve 80 to the working oil tank 17 is provided with an interchanger 86. A drain pipe 88 extending from the return spring 87 side of the switching valve 80 is connected to the working oil tank 17 side of the throttle valve 86 in the return pipe 85.

The hydraulic drive system 10 further includes a pilot pipeline 111 for discharging the first holding support pressure branched from the shrinking pilot pipeline 33 and a second retaining pipeline 111 branched from the pilot pipeline 53 for the open valve, The pressure in the pilot pipeline 112 for discharging the ground pressure and the pressure in the pilot pipeline 111 for discharging the first holding pneumatic pressure and the pressure in the pilot pipeline 112 for discharging the second holding pneumatic pressure, Pressure selection valve 113 (high-pressure-priority-type shuttle valve) for selectively leading to the pressure-receiving surface 89 of the switching valve 80.

That is, in a state where the pilot pressure is not generated by the opening / closing control valve 55, that is, when the operation lever 31a is not hardened and the operation lever 31a is in the state other than the load running, The valve position of the control valve 20 for the lift arm is shifted to the position where the pilot pressure for the lift arm is generated The valve position of the switching valve 80 is switched to the supplying position 80a by the return spring 87 when the valve position of the control arm for the lift arm is the first operating position 20a . In this state, when the operation lever device 31 generates the pilot pressure for shrinkage, that is, when the valve position of the lift-arm control valve 20 is the second operation position 20b, The pilot pressure for the first holding force is guided to the pressure receiving surface 89 of the switching valve 80 through the pilot line 111 for discharging the first holding force and the high pressure selecting valve 113, Is controlled to the discharge position 80b against the elastic force of the return spring 87. [

On the other hand, in the state in which the pilot pressure is generated by the opening / closing control valve 55, that is, in the load driving state, the pilot pressure generated by the opening / closing control valve 55 is transmitted to the pilot Is guided to the pressure receiving surface 89 of the switching valve 80 through the pipeline 112 and the high pressure selection valve 113 and the valve position of the switching valve 80 is switched to the discharge position 80b.

The operation of the hydraulic drive system 10 according to the present embodiment will be described in the following (1) to (4).

(1) The operation of the hydraulic drive unit 10 when the work device 2 is lifted while the wheel loader 1 is not traveling will be described first.

In this case, the operator of the wheel loader 1 operates the operation lever 31a to apply the extension pilot pressure to the lift-arm control valve 20 from the operation lever device 31, The valve position of the control valve 20 is switched to the first operating position 20a. The pump port 23 is connected to the bottom chamber side port 21 and the rod chamber side port 22 is connected to the tank port 24 in the control valve 20 for the lift arm.

In addition, the operator does not turn on the instruction switch 58. The vehicle speed detection signal outputted from the vehicle speed sensor 56 does not indicate the vehicle speed within the prescribed range at the time of load carrying. Therefore, the controller 57 does not apply the control current to the opening / closing control valve 55. [ That is, the open / close control valve 55 is not generating the pilot pressure.

In these states, the pilot pressure for shrinkage is not applied to the switching valve 80 through the pilot line 111 and the high-pressure selection valve 113 for discharging the first holding-support pressure, The pilot pressure is not applied to the switching valve 80 through the pilot line 112 for discharging and the high-pressure selection valve 113. [ As a result, the valve position of the switching valve 80 is controlled by the return spring 87 to the supply position 80a. The pressure in the bottom chamber 11a of the lift arm cylinder 11 is equal to the pressure in the portion of the bottom chamber side channel 15 interposed between the load holding valve 70 and the bottom chamber 11a, Side pressure chamber 76 through the first port 81 and the second port 82 and presses the poppet 71 together with the return spring 73 in the direction of closing the poppet 71. [ On the other hand, the discharge pressure of the main pump 13 is transmitted through the portion of the bottom chamber side passage 15 interposed between the control valve 20 for the lift arm and the load holding valve 70 to the first open side pressure chamber 74 So as to press the poppet 71 in the opening direction. The force for pushing the discharge pressure of the main pump 13 led to the first open side pressure chamber 74 in the opening direction of the poppet 71 is transmitted to the lift arm cylinder 11 The poppet 71 is pushed against the pressure in the bottom chamber 11a of the poppet 71 and the force that the return spring 73 presses the poppet 71 in the closing direction so that the load pressure holding valve 70 Is opened. That is, the load holding valve 70 permits the flow of the pressure oil toward the bottom chamber 11a of the lift arm cylinder 11 through the control valve 20 for the lift arm and the bottom chamber side conduit 15. The discharged oil of the main pump 13 passes through the pump port 23 and the bottom chamber side port 21 of the control valve 20 for the lift arm and then passes through the bottom chamber side channel 15, And supplied to the bottom chamber 11a of the cylinder 11. [ The pressurized oil in the load chamber 11b of the lift arm cylinder 11 passes through the rod chamber side conduit 16 and the rod chamber side port 22 and the tank port 24 of the control valve 20 for the lift arm And is discharged to the working oil tank 17. As a result, the lift arm cylinder 11 is stretched to lift the working device 2.

In this way, when the working device 2 is lifted up, that is, when the discharge oil of the main pump 13 flows through the control valve 20 for the lift arm and the bottom chamber side channel 15 to the bottom chamber of the lift arm cylinder 11 The valve position of the accumulator valve 61 is controlled to the accumulator pressure position 61a by the return spring 62 when the pressure in the accumulator 40 is less than the predetermined pressure. At this time, the accumulator valve 61 permits the flow of the pressure oil toward the accumulator 40 in the bypass pipeline 60 for the accumulation by the check valve 63, and blocks the flow of the pressure oil in the opposite direction . Therefore, a part of the pressure oil flowing in the bottom chamber side conduit 15 can flow until the pressure in the accumulator 40 reaches a predetermined pressure, or until the valve position of the control valve 20 for the lift arm reaches the first operating position 20a of the accumulator 40 through the bypass piping 60 for the accumulation. As a result, the accumulator 40 accumulates the pressure.

(2) The operation of the hydraulic drive apparatus 10 in the case where the work device 2 is held in an elevated state except during the load traveling will be described next.

In this case, since the operator of the wheel loader 1 does not operate the operating lever 31a, a pilot pressure for shrinking the pilot pressure for extension is not given to the lift-arm control valve 20 from the operating lever device 31 And the valve position of the control valve 20 for the lift arm is held at the neutral position 20c. As a result, both the pump port 23 and the bottom chamber side port 21 and the space between the pump port 23 and the rod chamber port 22 are blocked by the control valve 20 for the lift arm.

In addition, the operator does not turn on the instruction switch 58. The vehicle speed detection signal outputted from the vehicle speed sensor 56 does not indicate the vehicle speed within the prescribed range at the time of load carrying. Therefore, the controller 57 does not apply the control current to the opening / closing control valve 55. [ That is, the open / close control valve 55 is not generating the pilot pressure.

In these states, the pilot pressure for shrinkage is not applied to the switching valve 80 through the pilot line 111 and the high-pressure selection valve 113 for discharging the first holding-support pressure, The pilot pressure is not applied to the switching valve 80 through the pilot line 112 for discharging and the high-pressure selection valve 113. [ As a result, the valve position of the switching valve 80 is controlled by the return spring 87 to the supply position 80a. Therefore, the pressure in the bottom chamber 11a of the lift arm cylinder 11 is controlled by the bottom chamber side channel 15 interposed between the load holding valve 70 and the bottom chamber 11a, Side pressure chamber 76 through the port 81 and the second port 82 and presses the poppet 71 together with the return spring 73 in the direction of closing the poppet 71. [ The pressure in the bottom chamber 11a of the lift arm cylinder 11 is equal to the pressure of the portion of the bottom chamber side channel 15 interposed between the load holding valve 70 and the bottom chamber 11a of the lift arm cylinder 11. [ The second open side pressure chamber 75 is opened. Since the space between the main pump 13 and the load holding valve 70 is blocked by the control valve 20 for the lift arm, the pressure in the first opening side pressure chamber 74 of the load holding valve 70 The discharge pressure of the main pump 13 is not introduced. The pressure in the bottom chamber 11a of the lift arm cylinder 11 introduced into the closed side pressure chamber 76 and the return spring 73 are transmitted to the second open side pressure chamber 75 of the lift arm cylinder 11 The load pressure holding valve 70 is held in the closed state by pressing the poppet 71 against the valve seat 72 against the pressure in the bottom chamber 11a, do. That is, the load holding valve 70 is connected to the poppet 71 through the bottom chamber 11a of the lift arm cylinder 11 and through the bottom chamber side channel 15 to the control valve 20 for the lift arm, . As a result, the lift arm cylinder 11 is held in a stretched state to hold the working device 2 in an elevated state. The pressure oil in the bottom chamber 11a of the lift arm cylinder 11 is supplied to the working oil tank 17 through the gap of the control valve 20 for the lift arm in this state because it is the poppet 71, So that the work device 2 can be surely prevented from dropping.

(3) Operation of the hydraulic drive system 10 when the work device 2 is lowered while the wheel loader 1 is not running will be described.

In this case, the operator of the wheel loader 1 operates the operating lever 31a to apply a shrinking pilot pressure to the lift-arm control valve 20 from the operating lever device 31, And the valve position of the control valve 20 is switched to the second operating position 20b. The pump port 23 is connected to the rod chamber side port 22 and the bottom chamber side port 21 is connected to the tank port 24 in the control valve 20 for the lift arm.

In addition, the operator does not turn on the instruction switch 58. The vehicle speed detection signal outputted from the vehicle speed sensor 56 does not indicate the vehicle speed within the prescribed range at the time of load carrying. Therefore, the controller 57 does not apply the control current to the opening / closing control valve 55. [ That is, the open / close control valve 55 is not generating the pilot pressure.

In these states, the pilot pressure for shrinkage generated by the operation lever device 31 is supplied to the switching valve 80 through the pilot pipeline 111 for discharging the first holding pressure and the high-pressure selection valve 113, . As a result, the valve position of the switching valve 80 is switched to the discharging position 80b. The pressure in the closed side pressure chamber 76 of the load holding valve 70 is discharged to the working oil tank 17 through the second port 82 of the switching valve 80 and the tank port 83. [ On the other hand, the discharge oil of the main pump 13 passes through the pump port 23 and the rod chamber side port 22 of the control valve 20 for the lift arm, passes through the rod chamber side channel 16, Is supplied to the rod chamber (11b) of the rod (11). The pressure in the bottom chamber 11a of the lift arm cylinder 11 is introduced into the second open side pressure chamber 75 of the load holding valve 70 so that the poppet 71 is returned to the return spring 73 Against the elastic force, whereby the load holding valve 70 is opened. That is, the load holding valve 70 permits the flow of pressure oil from the bottom chamber 11a of the lift arm cylinder 11 to the control valve 20 for the lift arm through the bottom chamber side channel 15. The pressurized oil in the bottom chamber 11a of the lift arm cylinder 11 passes through the bottom chamber side port 15 and the bottom chamber side port 21 of the control arm 20 for the lift arm and the tank port 24, And is discharged to the tank 17. As a result, the lift arm cylinder 11 is contracted to lower the working device 2.

(4) The operation of the hydraulic drive system 10 when the wheel loader 1 is running, that is, when the wheel loader 1 is loaded, Explain.

In this case, since the operator of the wheel loader 1 does not operate the operating lever 31a, a pilot pressure for shrinking the pilot pressure for extension is not given to the lift-arm control valve 20 from the operating lever device 31 And the valve position of the control valve 20 for the lift arm is held at the neutral position 20c. As a result, both the pump port 23 and the bottom chamber side port 21 and the space between the pump port 23 and the rod chamber port 22 are blocked by the control valve 20 for the lift arm.

The operator turns on the indicating switch 58 and instructs the controller 57 to carry out the loading operation. When the vehicle speed detection signal outputted from the vehicle speed sensor 56 indicates the vehicle speed within the specified range at the time of loading, the controller 57 determines that the vehicle speed is within the specified range at the time of loading, As shown in Fig. Thereby, the opening / closing control valve 55 generates the pilot pressure. This pilot pressure is given to the on-off valve 50 through the pilot line 53 for the open valve and the valve position of the on-off valve 50 is switched to the open position 50b, And is supplied to the switching valve 80 via the pilot line 112 and the high-pressure selector valve 113 for switching the valve position of the switching valve 80 to the discharging position 80b.

In this state, the pressure in the closed-side pressure chamber 76 of the load holding valve 70 is transmitted to the hydraulic oil tank 17 through the second port 82 of the switching valve 80 and the tank port 83 . At this time, in the second open side pressure chamber 75, through the portion of the bottom chamber side channel 15 interposed between the load holding valve 70 and the bottom chamber 11a of the lift arm cylinder 11, The pressure in the bottom chamber 11a of the cylinder 11 is introduced. The pressure in the bottom chamber 11a of the lift arm cylinder 11 introduced into the second open side pressure chamber 75 pushes the poppet 71 against the elastic force of the return spring 73, The load holding valve 70 is opened. At this time, the valve position of the open / close valve 50 is controlled to the open position 50b, and the valve position of the control arm 20 for the lift arm becomes the neutral position 20c, The pressurized oil in the bottom chamber 11a of the lift arm cylinder 11 is released from the load retaining valve 70 and the connecting pipe 41 by opening the load retaining valve 70, To the accumulator (40). That is, the load holding valve 70 permits the flow of pressure oil from the bottom chamber 11a of the lift arm cylinder 11 to the accumulator 40 through the connecting pipe 41. [ As a result, the load pressure acting on the bottom chamber 11a of the lift arm cylinder 11 during the load traveling is absorbed by the accumulator 40, and the vertical movement of the working device is suppressed.

The current value I of the control current to the open / close control valve 55 gradually increases to I1 until the time T1 elapses after the vehicle speed of the wheel loader 1 falls within the specified range at the time of loading, Thereby, the opening / closing valve 50 gradually increases the opening degree (the position of the cutout in Fig. 3 and Fig. 4). Thereby, the pressure oil in the bottom chamber 11a of the lift arm cylinder 11 is prevented from being suddenly introduced into the accumulator 40, and the abrupt drop of the working device 2 accompanying the sudden inflow can be suppressed. The opening degree of the opening and closing valve 50 becomes sufficiently large when the time T1 elapses from the start of running of the wheel loader 1 and the pressure in the accumulator 40 is lower than the load pressure acting in the bottom chamber 11a of the lift arm cylinder 11 .

Further, when the valve position of the opening / closing valve 50 is controlled to the open position 50b, until the pressure in the accumulator 40 becomes approximately equal to the pressure in the bottom chamber 11a of the lift arm cylinder 11, The hydraulic oil flows from the bottom chamber 11a of the arm cylinder 11 to the accumulator 40 so that the lift arm cylinder 11 is contracted only by the amount of the pressure oil flowing into the accumulator 40. [ A portion of the discharge oil of the main pump 13 is supplied to the accumulator 40 when the working device 2 is lifted up as described in (1), so that the lift arm cylinder 11 is extended, 2, the wheel loader 1 is already loaded, and the oil pressure is accumulated in the accumulator. This makes it possible to reduce the amount of shrinkage of the lift arm cylinder 11, that is, the amount of descent of the working device 2 immediately after the valve position of the on-off valve 50 is controlled to the open position 50b.

According to the hydraulic drive system 10 of the present embodiment, the following effects can be obtained.

The hydraulic drive apparatus 10 according to the present embodiment can supply pressurized oil to the accumulator 40 through the accumulator 40 via the accumulator valve 61 when the working device 2 is lifted. This makes it possible to reduce the lower limit of the working device immediately after the valve position of the on-off valve 50 is controlled to the open position during the load traveling.

The hydraulic drive system 10 according to the present embodiment is configured such that the pressure oil in the bottom chamber 11a of the lift arm cylinder 11 is lifted by the lift The poppet 71 of the load retaining valve 70 can reliably prevent the drop of the working device from leaking to the working oil tank 17 through the gap of the control valve 20 for the arm. Thereby, it is possible to reliably prevent the working device 2 from descending against the intention of the operator.

The hydraulic drive system 10 according to the present embodiment is characterized in that the poppet 84 of the switching valve 80 is configured such that the flow of pressure oil from the first port 81 toward the tank port 83, Side pressure chamber 76 of the load retaining valve 70 without leaking the pressure in the bottom chamber 11a of the lift arm cylinder 11 because the pressure of the pressure oil toward the tank port 83 is prevented To act on the poppet 71 as shown in Fig. Therefore, the poppet 71 of the load retaining valve 70 can be held in the closed state, and thereby the pressure oil leaks from the load retaining valve 70 to the control valve 20 for the lift arm Can contribute to the prevention of things.

The hydraulic drive system 10 according to the above-described embodiment is applied to the work device 2 of the wheel loader 1, but may be applied to a front work device of a wheel shovel and a vessel of a dump truck.

1: Wheel loader (work vehicle)
2: working device
10: Hydraulic drive device
11: Lift arm cylinder (Hydraulic cylinder)
11a: Bottom thread
11b: Rod seal
13: Main pump
15:
16: rod side conduit
17: Working oil tank
20: Control valve for lift arm (control valve)
20a: First operating position
20b: second operating position
20c: Neutral position
21: Bottom port
22: Port on the load side
23: Pump port
24: Tank port
40: accumulator
41:
50: opening / closing valve
50a: Closed position
50b: open position
61: Pressure accumulator valve
61a: Axial pressure position
61b: closed position
70: Load holding valve
71: Poppet
73: return spring
74: first opening side pressure chamber
75: Second open side pressure chamber
76: Closed pressure chamber
80: Switching valve
80a: Supply location
80b: Discharge position
81: First port
82: Second port
83: Tank port
84: Poppet

Claims (2)

A hydraulic cylinder which supplies pressurized oil to the bottom chamber to raise the working device of the working vehicle and supplies the pressurized oil to the load chamber to lower the working device,
A main pump for discharging the pressure oil for driving the hydraulic cylinder,
A bottom chamber side port connected to the bottom chamber through a bottom chamber side channel, a rod chamber side port connected to the rod chamber through a rod side channel, a pump port connected to the main pump, and a tank port connected to the working oil tank A control valve including a spool valve,
An accumulator connected to an end of a connection channel branched from the bottom chamber side channel,
An on-off valve provided on the connecting pipe;
Closing valve is provided on an axial-pressure bypass line which is connected to the connection conduit and located beyond the on-off valve, and has an axial-pressure valve in parallel with the on-off valve,
Wherein the control valve has a first operating position which is a valve position connecting the pump port to the bottom chamber side port and connecting the rod chamber side port to the tank port, A second operating position that is a valve position for connecting the bottom chamber side port to the tank port and a neutral position for blocking both the pump chamber and the bottom chamber side port and between the pump chamber and the rod chamber side port In addition,
Wherein the on-off valve is capable of switching to an open position, which is a valve position for opening the connection channel and connecting the bottom chamber to the accumulator, and a closed position, which is a valve position for closing the connection channel and blocking the bottom chamber and the accumulator So that the valve position is controlled in the direction from the closed position to the open position when the working vehicle is running while the working vehicle is being elevated, , ≪ / RTI >
Wherein the accumulator valve includes an accumulator pressure position which is a valve position for permitting the flow of pressure oil toward the accumulator in the accumulator for bypassing the accumulator, Wherein the pressure in the accumulator is lower than a predetermined pressure, and the pressure in the accumulator is lower than a predetermined pressure. The valve position is controlled to the axial pressure position,
And a load holding valve is provided on the bottom chamber side conduit, and the load holding valve includes a poppet valve, and when the valve position of the control valve is in the first operating position, Permits the flow of pressurized oil toward the bottom chamber and permits the flow of pressurized oil from the bottom chamber to the control valve through the bottom chamber side channel when the control valve is in the second operating position, Wherein the control unit permits the flow of pressurized oil from the bottom chamber toward the accumulator through the connection channel, and when the valve position of the control valve is at a neutral position except during the loading travel, The flow of pressure oil toward the control valve is blocked by the poppet The hydraulic drive device for a working vehicle that. 2. The control valve according to claim 1, wherein the load retaining valve comprises: a return spring for urging the poppet in a direction to close the poppet; and a control unit for controlling the pressure in the direction in which the poppet opens against the elastic force of the return spring, A second opening side pressure chamber for introducing a pressure in a direction in which the poppet opens against the elastic force of the return spring from the bottom chamber side of the hydraulic cylinder, And a closing-side pressure chamber for introducing a hydraulic pressure for urging the valve in a direction to close the load-retaining valve and the bottom chamber in the bottom chamber side conduit from between the load holding valve and the bottom chamber,
And a switching valve is interposed between the bottom chamber and the closing chamber of the load holding valve. The switching valve is connected to the bottom chamber via the load holding valve and the bottom chamber A first port, a second port connected to the closed-valve-side pressure chamber, and a tank port connected to the working oil tank, wherein the first port is connected to the second port, and the first port, Wherein the control unit is configured to be capable of switching between a supply position that is a valve position for blocking the flow of pressure oil toward the tank port by the poppet and a discharge position that is a valve position for connecting the second port to the tank port, When the valve position of the valve is in the neutral position and when the valve position of the control valve is in the first operating position, And the valve position of the control valve is controlled to the discharge position when the valve is in the second operating position.

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